Much work has been done in both terrestrial and oceanic seismography in which reflections of acoustic waves (such as occur when incident acoustic waves strike an object of different density than the medium in which the wave is traveling) are analyzed to discern, identify and classify targets of interest. The sonar "ping" is a familiar example of a simple echo location system that may employ a circumnavigational wave for detecting undersea objects. In my prior U.S. Pat. No. 5,025,423, issued Jun. 18, 1991, I disclose a more sophisticated system, commercially known as the Enhanced Bottom Sonar System or EBSS. The EBSS identifies objects on the ocean floor by launching bursts of acoustic wave energy at different frequencies, collecting the reflected signals, and comparing the collected data with a library of data sets, each set corresponding to the back-scatter energy of a particular type of object. However, this system requires an array having as many as 25 transmitting transducers, which makes it too unwieldy for locating shore-based underground objects such as utility pipes. My co-pending provisional application entitled "Ground Penetrating Sonar", Ser. No. 60/015,859 filed Jul. 8, 1996, discloses a parametric transmitting transducer which, advantageously can launch a near-field, narrow acoustic beam with negligible side lobes using just a single parametric transducer. It would be extremely advantageous to further improve the last mentioned system by permitting the continuous reception of echoes as the receiver is moved over the ground.